A need for very fast, continuously tunable sources over wide spectral range has been recognized in sensing and process control applications. As an illustration of a technical challenge, consider cycle-resolved, MHz-scale resolution of combustion process in a modern jet engine that operates at or above 100,000 rpm, generating spectral signatures over hundreds of nanometers across the optical (infrared) range. While the capture of the entire spectral content might be demanding, even a partial, 100 nm-wide acquisition requires a MHz-linewidth source capable of ˜106 nm/s tuning (sweep) rate. Similar challenge is posed by the use of tunable source for biomedical imaging purposes. A video-rate optical coherence tomography (OCT) with sub-micron resolution would dictate ˜109 nm/s sweep rate over spectral range measured in hundreds of nanometers, which is currently out of reach of conventional tunable laser physics. Continuously tunable oscillator devices have been introduced both in RF and optical domains and rely on diverse technologies.
A combination of fundamental and practical barriers sets strict limits on laser tuning performance. An ideal tunable source should possess unlimited tuning speed, be capable of sweeping over arbitrarily wide band while attaining narrow line-width. Unfortunately, the existence of such source is prohibited by the Heisenberg principle, as its operation would imply that the strict spectral localization, inferred by narrow linewidth, is possible in spite of arbitrarily fast tuning speed and range. In a limit, an infinite sweep rate would imply that the source frequency can be anywhere within the tuning range while the near-zero linewidth would guarantee exact spectral localization.
Long before reaching the uncertainty limit, fundamental laser physics would have barred one from constructing fast and coherent tunable source. All known tunable technologies rely on cavity reconfiguration to achieve wavelength tuning, in which the cavity length is varied to enforce the new resonant frequency. Even though the cavity can be reconfigured swiftly, either via mechanical, optical or electrical means, the linewidth is invariably sacrificed since the coherence of the laser wave depends on the photon cavity lifetime. Indeed, in order to tune to the new wavelength, one has to wait longer than the photon cavity lifetime. Worse, the photon lifetime is inversely proportional to the source linewidth: a narrower linewidth implies longer cavity reconfiguration time, implying that highly resonant cavities are not amenable to swift reconfiguration. The latter fact represents the true limit of the tunable technology that cannot be circumvented, even in principle, by any cavity engineering techniques. Consequently, the tunable source can be engineered for either tuning speed or narrow linewidth, but not for both simultaneously.
Continuously tunable, single wavelength sources are typically constructed by rapid reconfiguration of optical cavity and appear in multiple forms that include mechanical, electrical and all-optical techniques. An ideal tunable source has large tuning speed (Δλ/Δt), wide spectral range (αλ) and narrow spectral width (δλ). A general figure of merit (M) quantifies the laser agility and is defined as a product of the tuning range, the tuning speed and the inverse of the source linewidth:
                    M        =                  Δ          ⁢                                          ⁢          λ          ×                      Δλ                          Δ              ⁢                                                          ⁢              t                                ×                                    δλ                              -                1                                      .                                              (        1        )            
The construction of rapidly tunable, arbitrarily narrow linewidth source is both fundamental and practical challenge. To illustrate this, consider the rapid tuning requirements: the tunable cavity reconfiguration (and stabilization) time needs to be comparable to the photon roundtrip time; a narrow source linewidth (δλ) necessarily implies a long roundtrip time, in direct contradiction to the requirement for fast cavity changes. In practical terms, fast, repeatable cavity reconfiguration poses significant engineering challenge over wide spectral ranges.